OVEREXPRESSION OF THIOREDOXIN BINDING PROTEIN (TBP-2) INCREASES OXIDATION SENSITIVITY AND APOPTOSIS IN HUMAN LENS EPITHELIAL CELLS

Thioredoxin (Trx) is an important redox regulator with cytosolic Trx1 and mitochondrial Trx2 isozymes. Trx has multi-physiological functions in cells and its bioavailability is negatively controlled through active site binding to a specific thioredoxin binding protein (TBP-2). This paper describes the delicate balance between TBP-2 and Trx, and the effect of overexpression of TBP-2 in the human lens epithelial cells. Cells overexpressing TBP-2 (TBP-2 OE) showed a 7-fold increase of TBP-2, and a nearly 40% suppression of Trx activity but no change in Trx expression. The TBP-2 OE cells grew slower and their population decreased to 30% by day 7. Cell cycle analysis showed that TBP-2 OE cells arrested at the G2-M stage, and that they displayed low expressions of the cell cycle elements P-cdc2 (Y15), cdc2, cdc25A and cdc25C. Furthermore, TBP-2 OE cells were more sensitive to oxidation. Under H 2 O 2 (200 µM, 24 hrs) treatment, these cells lost 80% viability and became highly apoptotic. Brief oxidative stress (200 µM, 30 min) to TBP-2 OE cells disrupted the Trx anti-apoptotic function by dissociating the cytosolic and mitochondrial Trx-ASK binding complexes. The same H 2 O 2 -treated cells also showed activated ASK (P-ASK), Bax, lowered Bcl2, cytochrome c release, and elevated caspase 3/7 activities. We conclude from these studies that high cellular levels of TBP-2 can potentially suppress Trx bioavailability and increase oxidation sensitivity. Overexpression of TBP-2 also causes slow growth by mitotic arrest, and apoptosis by activating the ASK death pathway.